Thermodynamic machines, such as energy transfer or heat pump systems using a Stirling, Vuilleumier or other cycle, supply thermal energy, mechanical energy, or refrigeration capacity for external use. A particularly advantageous form of such systems for many heat transfer applications is described in a previously filed application of the present inventor entitled "Unitary Heat Engine/Heat Pump System", Ser. No. 335,659, filed Dec. 30, 1981. In this integrated system, thermal energy output is developed at an intermediate temperature level (e.g. 60.degree. C.-100.degree. C.) with high coefficient of performance. Because the system uses ambient temperature thermal energy as an input (e.g. solar energy, water, ground, or waste energy sources) it is particularly suitable for large (i.e. more than 100 KW) installations. Consequently it is also particularly suitable for large central heating installations.
Most current thermodynamic machines employ a thermal regenerator the conduit for a working fluid being transferred back and forth between a hot working chamber and a cold working chamber. Each of the chambers has a cyclically driven displacer or piston for establishing the pressure-volume changes used in the particular thermodynamic cycle. Thermal regeneration forms a vital part of the process by accepting thermal energy from and returning it back to the working fluid. For heating and refrigeration applications, the working fluid of the cyclically varying thermodynamic system must at some point undergo thermal interchange with an external device or fluid. In the system described in patent application Ser. No. 335,659 mentioned above, for example, heat exchanger couplings may be used in different combinations at the cold, intermediate and hot temperature levels. In a refrigeration system using solar energy input, for example, heat exchangers are used at both the hot and cold ends of the machine to couple working fluid to an external medium. The heat exchanger, usually but not necessarily a countercurrent device, enables efficient access of an exterior source or system to the working fluid within the system. It does, however, introduce a penalty by significantly enlarging the interior volume to be occupied by the working fluid. Thus the "dead space" in the system may be significantly increased, with consequent deterioration of power output and efficiency for systems of the Vuilleumier type. As defined by Walker in the book "Stirling-Cycle Machines", Clarendon Press, Oxford, 1973, pp. 19-20, the dead space for Stirling devices is "that part of the working space not swept by one of the pistons, and includes cylinder clearance spaces, void volumes of the regenerator and other heat-exchangers, and the internal volume of associated ducts and ports." As shown by Walker, supra, at p. 40, there is a significant drop-off in specific power (expressed as a power parameter) or specific heat load with increasing dead space (expressed as a dead-space ratio) for given temperature ratio, swept-volume ratio, and piston lead angle for the Stirling. The same general relationship is recognized as holding true for other thermodynamic cycles as well. In the unitary heat engine/heat pump system, in which three such heat exchangers are used and system performance is strongly affected by thermodynamic considerations, increased dead space can have a strong adverse effect on system performance.
The use of heat exchangers that interchange thermal energy with the working fluid and external systems has heretofore been regarded as introducing unavoidable dead space in thermodynamic machines. The degree of the penalty imposed is unfortunately increased when a system is intended to operate with available, relatively low temperature heat sources, such as solar energy or waste heat, or to provide significant amounts of thermal energy output (e.g. greater than 100 KW) at intermediate temperature levels. In such situations the pressure ratio and temperature ratio of the thermodynamic machine may be relatively low at the outset and the penalty imposed by increasing the dead space may be excessive.